This is very good news if true! One of the biggest strikes against Banjo & Kazooie as a playable character in the base game was if the 103 stage count was final (because I feel like every third party character should come with a stage). Of course, it's good to be cautious--always possible it was a misprint.
My speculation for what the stages are, if true:
104. Sun and Moon stage (if Incineroar is playable)
105. Golden Sun stage (if Isaac is playable)
106. Banjo-Kazooie stage (if Banjo & Kazooie are playable, probably the top of Grunty's Tower with Gruntilda as a stage boss)
107. Monster Hunter stage (makes sense, if Dracula's Castle has a solo version and multiplayer version--probably no MH character)
108. Minecraft stage (with a Creeper assist trophy, as Ratholos was a third party assist with a stage, lines up with Verge)
My reasoning is that the remaining stages would be new (no returning stages), based on new series (with the exception of Pokemon, because it's Pokemon), and new series third party assists would get their own stage. Geno, Skull Kid, Bandana Waddle Dee, Dixie (if unique), Shadow (if unique) could all come with no new stages, though Geno also could since he's technically third party (he's in a weird nebulus area, being a third party Mario character). Only thing I see being in the mix is if the rumored Bandai Namco character is true, in which case we may get a Tales of Symphonia stage (I don't think Heihachi would be a base game character). As much as I would love Dante, the creator said a clear no, and I don't think Capcom would have three characters, with the rest at 2.
It would be nice to have Pac-Maze back, a new Sonic stage from SA2, a new Pokefloats with Pokemon from every generation. But, I would be completely satisfied if this was the final stage list, consistent with Verge's claims. I still think 15 new stages is more reasonable for a new game, but 9 new stages is much better than only 4 new stages. Still remaining skeptical, but optimistic.